[cmake][python] allow installing Python package from build directory

For packages building Python modules from C++ code, the package cannot be installed from the source directory as it must contain the generated shared objects. This patch allows installing from the build directory instead.

To install from the build directory, the developer places a setup.py.in in the directory containing the source of the python package instead of a setup.py. This file is automatically configured by build system. Python files, however, have to be copied to the build directory by hand, e.g., using dune_symlink_to_source_files.